An input layout requires the input signature of the vertex shader you're going to use. I'm not too familiar with SlimDX, but it looks like you're using the ShaderSignature class wrong by giving it a reference to your vertex buffer.

Giving the shader signature a data stream that contains your vertex buffer doesn't make any sense, as it's just a chunk of bytes. The signature isn't data, it specifies how that data is laid out per vertex. The shader signature needs to be taken from the shader you'll be using.

E.g. Using the effect classes, each effect pass will have a signature (for the vertex shader used) in its effect description you can use to create the input layout.

You can re-use these layouts with other shaders that have identical signatures.